1. Field of the Invention
The present invention relates to a ring laser gyro for detecting rotation using a semiconductor ring laser.
2. Related Background Art
Conventionally, as gyros for detecting rotation, that is, angular velocity, of an object, mechanical gyros having a rotor and an oscillator, optical gyros, and the like are known. Since optical gyros can be instantaneously activated and have a wide dynamic range, they are making innovations in the field of gyros. Optical gyros can be categorized into ring laser gyros, optical fiber gyros, passive ring resonator gyros, and the like. Ring laser gyros using gas lasers have already been put into practical use in the field of airplanes. As small-sized ring laser gyros with high accuracy, gyros formed of semiconductor ring lasers have been proposed, which are disclosed in Japanese Patent Publication Nos. 62-39836, 6-38529 and Japanese Patent Application Laid-Open No. 4-174317.
A gyro formed of a semiconductor ring laser over a mechanical gyro having an oscillator are characterized by a smaller size, lower power consumption, and shortened activating time, which makes the gyro suitable for use as a gyro stabilizer in a still camera or a video camera for preventing blurred image due to movement of the camera.
In such a gyro, beat frequency has information on angular velocity. In order to detect the beat frequency, there is a method of converting the beat frequency to a voltage signal via a frequency-voltage conversion circuit, a method of directly detecting the beat frequency through a frequency counter, and the like.
However, a conventional gyro formed of a semiconductor ring laser can not detect the rotational direction from an output signal as it is. Therefore, minute rotational vibration (dithering) is added to detect the rotational direction from the correlation between the dithering and the signal. The Japanese Patent Publication No. 62-39836 and the Japanese Patent Application Laid-Open No. 4-174317 do not disclose the method of detecting the rotational direction.
A conventional ring laser gyro is required to improve the accuracy of detecting a beat signal.
In order to attain the above object, a ring laser gyro according to the present invention comprises at least one pair of semiconductor ring lasers, the semiconductor ring lasers being optically independent of each other and variation in a period of an impedance change in one of the pair being opposite to that in the other with respect to rotation in one direction. More specifically, a ring laser gyro comprising two or more semiconductor ring lasers, the semiconductor ring lasers being optically independent of each other, is characterized in that a change in beat frequency with respect to a change in angular velocity of a first ring laser is opposite to that of a second ring laser, and angular velocity of rotation of the gyro is detected by a signal representing a difference between a first beat frequency generated by the first ring laser and a second beat frequency generated by the second ring laser. In particular, the ring laser gyro is characterized in that the beat frequency generated by the first ring laser and the beat frequency generated by the second ring laser in a static state are equal to each other, and the rate of change in the beat frequency with respect to the change in the angular velocity of the first ring laser is equal to that of the second ring laser.
A ring laser gyro according to the present invention is provided with two semiconductor ring lasers, the semiconductor ring lasers being in planes nonperpendicular to each other, being optically independent of each other, and each being provided with an electric terminal for detecting an impedance change, a period of the impedance change between the terminals being varied according to applied angular velocity. When angular velocity in a direction is increased, frequency of the impedance change with respect to the first semiconductor ring laser is decreased, while frequency of the impedance change with respect to the second semiconductor ring laser is increased.
In the above structure, by carrying out signal processing with respect to a frequency change in the impedance change of the two semiconductor ring lasers, a signal which depends on the angular velocity can be separated from a change in the beat frequency in a static state and noises. The sign of variation in the signal which depends on the angular velocity of one semiconductor ring laser is opposite to that of the other semiconductor ring laser, while the sign of variation in the change in the beat frequency in a static state and the noises of one semiconductor ring laser is the same as that of the other semiconductor ring laser. In this way, the S/N ratio can be improved, and thus, the angular velocity and the rotational direction can be obtained with accuracy from a signal which depends on the angular velocity.
According to the present invention, in the ring laser gyro, each of the two semiconductor ring lasers has two laser beams, the two laser beams rotating in opposite directions in their respective optical resonators and having different frequencies of oscillation in a static state. The relationship in magnitude between the frequency of oscillation of the clockwise laser beam and the frequency of oscillation of the counterclockwise laser beam is reversed between the two semiconductor ring lasers.
In the above structure, the two laser beams of each of the two semiconductor ring lasers rotate in opposite directions in their respective optical resonators and have different frequencies of oscillation in the static state generate optical beat. Further, since the two semiconductor ring lasers are optically independent of each other, when the gyro provided with them is rotated, the frequencies of oscillation of the respective laser beams vary independently of each other. Since the relationship in magnitude between the frequency of oscillation of the clockwise laser beam and the frequency of oscillation of the counterclockwise laser beam is reversed between the two semiconductor ring lasers, when the gyro is rotated, the optical beat frequency in the optical resonator of one semiconductor ring laser is increased while that of the other is decreased. This variation in the optical beat frequencies is detected as a frequency change in the impedance change between the terminals of the semiconductor ring lasers.
By carrying out signal processing with respect to the obtained frequency change, a signal which depends on the angular velocity can be separated from a change in the beat frequency in a static state and noises. The sign of variation in the signal which depends on the angular velocity of one semiconductor ring laser is opposite to that of the other semiconductor ring laser, while the sign of variation in the change in the beat frequency in a static state and the noises of one semiconductor ring laser is the same as that of the other semiconductor ring laser. In this way, the S/N ratio can be improved, and thus, the angular velocity and the rotational direction can be obtained with accuracy from a signal which depends on the angular velocity.
Further, according to the present invention, in the ring laser gyro, the two semiconductor ring lasers have a tapered portion in a part of their respective optical waveguides. The tapered portion is formed of a first portion where the width of the optical waveguide becomes larger along a propagation direction of a clockwise laser beam and a second portion where the width of the optical waveguide becomes smaller. In the first semiconductor ring laser, the first portion is longer than the second portion, while, in the second semiconductor ring laser, the second portion is longer than the first portion.
Here, the tapered portions of the semiconductor ring lasers have structures introduced for the purpose of giving difference between the frequency of oscillation of the clockwise laser beam and the frequency of oscillation of the counterclockwise laser beam in the static state. Further, the relationship of which is longer between the first portion and the second portion of the tapered portion is reversed between the first and the second semiconductor ring lasers. This makes the dependence of resonator loss on the rotational direction reversed between the first and the second semiconductor ring lasers. Therefore, the relationship in magnitude between the frequency of oscillation of the clockwise laser beam and the frequency of oscillation of the counterclockwise laser beam is reversed.
Here, more specifically, the tapered portions function as follows. The laser beam in the optical resonator is propagated repeating total internal reflections at the interface of the optical waveguide. Since, at the tapered portion, the angle of incidence upon the interface of the optical waveguide does not satisfy the total internal reflection condition, waveguide loss is caused. Since the angle of incidence upon the interface at the tapered portion differs depending on the rotational direction, the loss also differs, i.e., the resonator loss depends on the rotational direction. The difference in the resonator loss makes a difference in the oscillation threshold value of the ring laser. When two laser beams of different rotational directions coexist and oscillate, the photon density differs. The difference in the photon density makes different the frequency of oscillation of the laser beam due to nonlinear effect.
Since, in each of the semiconductor ring laser, the frequency of oscillation of the clockwise laser beam and that of the counterclockwise laser beam differ, optical beat is generated. Further, since the two semiconductor ring lasers are optically independent of each other, when the gyro provided with them is rotated, the frequencies of oscillation of the respective laser beams vary independently of each other. Since the relationship in magnitude between the frequency of oscillation of the clockwise laser beam and the frequency of oscillation of the counterclockwise laser beam is reversed between the two semiconductor ring lasers, when the gyro is rotated, the optical beat frequency in the optical resonator of one semiconductor ring laser is increased while that of the other is decreased. This variation in the optical beat frequencies is detected as a frequency change in the impedance change between the terminals of the semiconductor ring lasers.
By carrying out signal processing with respect to the obtained frequency change, a signal which depends on the angular velocity can be separated from a change in the beat frequency in a static state and noises. The sign of variation in the signal which depends on the angular velocity of one semiconductor ring laser is opposite to that of the other semiconductor ring laser, while the sign of variation in the change in the beat frequency in a static state and the noises of one semiconductor ring laser is the same as that of the other semiconductor ring laser. In this way, the S/N ratio can be improved, and thus, the angular velocity and the rotational direction can be obtained with accuracy from a signal which depends on the angular velocity.
According to the present invention, in the ring laser gyro, the ratio of the area surrounded by the resonator to the length of a revolution of the resonator in the first semiconductor ring laser is equal to that in the second semiconductor ring laser.
In the above structure, the ratio of the area surrounded by the resonator to the length of a revolution of the resonator in each of the semiconductor ring laser is a parameter which determines the absolute value of the amount of variation in the beat frequency with respect to the change in the angular velocity. When this parameter is the same between the two semiconductor ring lasers, the amounts of variation in the beat frequencies of the two semiconductor ring lasers have the same absolute value and the opposite polarities. Therefore, it becomes still easier to separate a signal which depends on the angular velocity from a change in the beat frequency in a static state and noises. The sign of variation in the signal which depends on the angular velocity of one semiconductor ring laser is opposite to that of the other semiconductor ring laser, while the sign of variation in the change in the beat frequency in a static state and the noises of one semiconductor ring laser is the same as that of the other semiconductor ring laser. In this way, the S/N ratio can be improved, and thus, the angular velocity and the rotational direction can be obtained with more accuracy from a signal which depends on the angular velocity.
Further, in the ring laser gyro, the shapes of the resonators of the first and the second semiconductor ring lasers have mirror symmetry each other.
In the above structure, since the shapes of the resonators of the first and the second semiconductor ring lasers are mirror images of each other, the dependence of the resonator loss on the rotational direction is made to be symmetrical mirror image between the two semiconductor ring lasers. In other words, the rotational loss of the clockwise laser beam in the first semiconductor ring laser is the same as the rotational loss of the counterclockwise laser beam in the second semiconductor ring laser, and the rotational loss of the counterclockwise laser beam in the first semiconductor ring laser is the same as the rotational loss of the clockwise laser beam in the second semiconductor ring laser. It follows that, when the driving conditions are the same between the two semiconductor ring lasers, the beat frequency in the static state is also the same. The beat frequency in the static state is independent of the angular velocity during rotation. When the value is the same between the two semiconductor ring lasers, a signal component dependent on the angular velocity can be separated with high accuracy by carrying out subtraction between signals from the two semiconductor ring lasers. Since, by the subtraction between the two signals, a noise component independent of the angular velocity is also suppressed, the S/N ratio can be improved. As a result, the angular velocity and the rotational direction can be obtained with accuracy.
Further, according to the present invention, the optical laser gyro comprises an absorber or a light-shield for preventing optical coupling between the two semiconductor ring lasers, and the absorber or the light-shield is prevented from returning reflected light to the semiconductor ring lasers.
In the above structure, the absorber or a light-shield for preventing the optical coupling makes it possible for the respective semiconductor ring lasers to carry out laser oscillation optically independently of each other. Further, by preventing the absorber or the light-shield from returning reflected light to the semiconductor ring lasers, optical coupling between the clockwise laser beam and the counterclockwise laser beam is prevented to suppress lock-in.
When the gyro formed of the two optically independent semiconductor ring lasers structured in this way is rotated, the frequency of oscillation of the clockwise laser beam and the frequency of oscillation of the counterclockwise laser beam change independently from each other in each of the semiconductor ring laser. By carrying out signal processing with respect to them, the angular velocity can be obtained. Therefore, a ring laser gyro which operates with stability in a wide range of angular velocity can be realized.
Driving methods such as constant voltage drive and constant current drive make it possible to take out an impedance change of a device with a simple circuit structure, and connection to various signal processing circuits can be realized with ease. In a signal processing circuit, a signal which depends on the angular velocity can be separated from a change in the beat frequency in a static state and noises. The sign of variation in the signal which depends on the angular velocity of one semiconductor ring laser is opposite to that of the other semiconductor ring laser, while the sign of variation in the change in the beat frequency in a static state and the noises of one semiconductor ring laser is the same as that of the other semiconductor ring laser. In this way, the S/N ratio can be improved, and thus, the angular velocity and the rotational direction can be obtained with accuracy from a signal which depends on the angular velocity.
Further, according to the present invention, in a method of driving a gyro, current injected to or voltage applied to the two semiconductor ring lasers is the same.
In the above structure, the same injected current and the applied voltage lead to small differences in the oscillation frequencies, the light intensities, and the generated heats between the first and the second semiconductor ring lasers, which result in the same beat frequency in the static state. This driving is particularly useful when there is mirror symmetry between the shapes of the two semiconductor ring lasers. By driving in this way, the beat frequency in the static state, which is independent of the angular velocity during rotation, becomes common in the respective semiconductor ring lasers. Therefore, a signal component dependent on the angular velocity and the beat frequency in the static state independent of the angular velocity can be separated with ease and with high accuracy, which makes it possible to improve the S/N ratio. As a result, the angular velocity and the rotational direction can be obtained with accuracy from a signal dependent on the angular velocity.
Further, according to the present invention, a signal processing is carried out with respect to the frequency of the impedance change in the two semiconductor ring lasers to obtain the angular velocity and the rotational direction.
In the above structure, the signal processing separates a signal dependent on the angular velocity and a beat frequency component in the static state, which is independent of the angular velocity.
Here, the signal, which depends on the angular velocity, is separated from a change in the beat frequency in the static state and noises. The sign of variation in the signal, which depends on the angular velocity of one semiconductor ring laser is opposite to that of the other semiconductor ring laser, while the sign of variation in the change in the beat frequency in a static state and the noises of one semiconductor ring laser is the same as that of the other semiconductor ring laser. In this way, the S/N ratio can be improved, and thus, the angular velocity and the rotational direction can be obtained with more accuracy from a signal, which depends on the angular velocity.
Further, according to the present invention, a signal processing in the ring laser gyro is subtraction or negatively weighted average.
In the above structure, the difference in the frequencies of the impedance change obtained by the subtraction is in proportion to the angular velocity including the sign representing the rotational direction. This signal processing is particularly beneficial when the beat frequency is substantially the same between the two semiconductor ring lasers in the static state under the same driving conditions and the amounts of variation in the beat frequencies with respect to variation in the angular velocity of the two semiconductor ring lasers have the same absolute value and the opposite polarities.
By the negatively weighted average, a signal component due to the beat frequency in the static state, which is independent of the angular velocity, is canceled out and a signal in proportion to the angular velocity including the sign representing the rotational direction is obtained. This signal processing is particularly valuable when the beat frequencies in the static state of the two semiconductor ring lasers are not the same.
The weighting may be carried out as follows. Reciprocals of the frequencies of the impedance change in the static state of the two semiconductor ring lasers are respectively found, and one of the reciprocals multiplied by (xe2x88x921) is used as the weight of the weighted average.
Here, the signal which, depends on the angular velocity, is separated from a change in the beat frequency in the static state and noises. The sign of variation in the signal, which depends on the angular velocity of one semiconductor ring laser, is opposite to that of the other semiconductor ring laser, while the sign of variation in the change in the beat frequency in a static state and the noises of one semiconductor ring laser is the same as that of the other semiconductor ring laser. In this way, the S/N ratio can be improved, and thus, the angular velocity and the rotational direction can be obtained with more accuracy from a signal which depends on the angular velocity.
Further, according to the present invention, the ring laser gyro is drived with respect to the frequencies of the impedance change in the respective semiconductor ring lasers, and the driving conditions are controlled by using the result of the signal processing on the frequencies of the impedance change.
In the above structure, the signal processing with respect to the frequencies of the impedance change in the respective semiconductor ring lasers separate a signal independent of the angular velocity. By controlling the driving conditions to stabilize the signal, which is independent of the angular velocity over time, a signal, which is dependent on the angular velocity is, separated with suppressing the change in the signal over time, leading to high accuracy of separation.
Further, according to the present invention, the driving method of the ring laser gyro is characterized in that the operation is addition or weighted average.
In the above structure, the addition or the weighted average separates a component independent of the angular velocity from a signal obtained from the respective semiconductor ring lasers. In particular, in the two semiconductor ring lasers, when the absolute values of the frequencies of the impedance change with respect to the angular velocity are the same and their signs are opposite, a component independent of the angular velocity is obtained by the addition. When, in the two semiconductor ring lasers, the absolute values of the frequencies in the impedance according to the angular velocity are not the same, the absolute values of the frequencies are differentiated with respect to the angular velocity, and the reciprocals of the results are used as the weight of the weighted average. By controlling the driving conditions to stabilize the signal obtained in this way, which is independent of the angular velocity over time, the change in the signal over time can be decreased, and thus, the accuracy can be improved when a signal dependent on the angular velocity is separated.